The basic physical principle of the heat-to-electricity conversion employed by the invention is the Seebeck effect, whereby a temperature difference produces a voltage across a p-n junction. This is essentially the inverse of the Peltier effect whereby an applied voltage can be used to generate either cooling or heating, depending on the polarity of the voltage. This process involves the diffusion transport of electrons and holes. Highly efficient doped bulk alloy semi-conductors, that are operated at elevated temperatures and can be machined or fabricated to meet practical design requirements without complicated manufacturing processes, make this type of TE device appealing for direct electricity generation in both terrestrial and space applications. The underlying physics is thought to be an enhancement of electrical conductivity via the doping agent by increasing the density of electron states, which increases the Seebeck coefficient, without increasing the thermal conductivity of the semi-conductor. The invention provides a method to take advantage of doped bulk alloy materials that have high figures-of-merit and therefore high electrical conversion efficiencies at elevated temperatures. Other types of TE devices, such as nano-structured semiconductors, can also be used in the invention.
The theoretical efficiency ηte of a TE device based on the Seebeck effect can be expressed as the product of two factors, ηte=ηcar*(ηte/ηcar). A high device efficiency ηte requires a high Carnot efficiency, ηcar=ΔT/Thot, which implies that a large temperature difference, ΔT=Thot−Tcold, where Thot is the TE hot side temperature and Tcold is the cold side TE temperature, is desirable across the device for a given value of Thot. The second factor ηte/ηcar in the expression for ηte is the fraction of Carnot for TE devices and it is a nonlinearly increasing function of ZT, the dimensionless figure-of-merit for the TE device, where Z itself may be a function of temperature. The figure-of-merit Z is defined as Z=α2σ/k, where α is the Seebeck coefficient, σ is the electrical conductivity, and k is the thermal conductivity of the device material. The factor ηte/ηcar is commonly presented as ηte/ηcar=[(M−1)/(M+1−ηcar)], where M=(<ZT>+1)^0.5. In determining the maximum TE device efficiency ηte, as the device figure-of-merit varies over the module temperature range Tcold<T<Thot, the integrated average value of ZT for both the n-doped and p-doped components, <ZT>, is the appropriate figure-of-merit for estimating the device efficiency. For a given value of war, an increase in <Z> by a factor of two (for example, Z=0.002/K→Z=0.004/K) can result in a 50% increase in ηte.
The ratio ηte/ηcar can be expressed explicitly in terms of <ZT> and ηcar, and it turns out to be a relatively weaker function of ΔT than does ηcar, which is directly proportional to ΔT. Because the figure-of-merit Z is highest at the highest temperatures, a small ΔT in the upper temperature range maximizes <ZT>, but since ηcar is the more dominant factor in the product ηcar*(ηte/ηcar), the optimum ΔT to maximize the efficiency ηte turns out to be its largest practical value for any given value of Tcold. If contact thermal resistance is minimized, the value of ΔT depends on the product of the TE device total (intrinsic and contact) thermal resistance Rte and the conducted heat flow Qcond, which depends on the magnitude of the solar flux and ηte, so that Thot=Tcold+Rt*Qcond. Therefore, it follows that the optimum value of Tcold should be its lowest practical value to maximize ηcar and ηte. An additional operational constraint is that Thot must not exceed a maximum temperature Tcrit that could damage the TE device. The invention provides an arrangement of passive and active mechanisms to achieve the lowest practical value of Tcold, consistent with a terrestrial or space environment, and the largest value of ΔT, such that Thot<Tcrit. Given the potential variation of the heat input source, i.e., the solar flux, ΔT may be limited by the maximum available heat flow Qcond. In this case the invention Numerical Design Model provides the optimum design and operating mode to maximize ηte. Alternatively, the invention Numerical Design Model can be programmed to optimize other desirable results, such as minimum C-STEPS mass, as in a space application, while retaining acceptable values of ηcar and ηte.
Previous work on the invention concept was conducted by the inventor beginning in December 2006 under contract to Broad Reach Engineering, Golden Colo. Several Small Business Innovative Research (SBIR) proposals for funding the development of the invention have been submitted by the inventor to NASA and other U.S. Government agencies since December 2006, although no development funding has been obtained as of the date of this application.
Direct solar heating of TE devices, with and without solar concentration, is discussed by H. J. Goldsmid in Applications of Thermoelectricity, John Wiley and Sons, New York, 1960, pages 112-114. The concept of a reflector/radiator component is not shown or discussed as a method to direct solar flux onto the TE module hot side nor to remove the unconverted conducted heat from the TE module cold side.
Only one U.S. patent could be identified that describes methods for the production of both electrical and thermal energy using a solar energy source in combination with TE devices:    US 2010/0186794A1 Chen et al Jul. 29, 2010
Form 8A, Information Disclosure Statement by Applicant, is attached herein to request inclusion of said patent in the examination process. The following comparisons, design differences, and innovations in regard to the invention described herein are noted:
A dual reflector system is shown in FIG. 10C of patent US 2010/0186794A1 for the purpose of heating the hot side of a TE module located in the vicinity of the central axis of the primary concave reflector. Such a dual reflector system is not a new concept, as it is used in a variety of optical instruments, including Baker-Schmidt cameras, Kellner-Schmidt auto-collimators, and Schmidt-Cassegrain telescopes.
In contrast to the design in FIG. 10C, and its description on page 12 of patent US 2010/0186794A1, and in contrast to the functions of the optical instruments cited in the preceding paragraph, the C-STEPS design for redirecting the solar energy back toward the center of the primary concave reflector/radiator is to permit said reflector/radiator to function as an IR radiator for rejecting some fraction of the conducted heat flowing into the cold side of the TE module, which is mechanically and thermally integrated with the reflector/radiator in the preferred embodiment of the invention. This dual function allows the C-STEPS invention to operate efficiently, especially in a space environment, e.g., on a spacecraft or satellite, in which convection or conduction mechanisms are typically undesirable or impractical for reducing the cold side TE temperature. The dual function of the reflector/radiator as both a solar collector for the TE module hot side heat input, and an IR radiation energy rejection mechanism for the TE cold side excess heat, is a primary differentiating feature of this invention.
Secondly, the extraction of heat from the cold side of the TE module, whether redirected for passive thermal heating or for heat engine electricity production, or neither, is not an innovative or original concept; it is a necessity for basic operation of any TE module by establishing a temperature difference across the TE module. Without such heat removal, whether by design or not, the TE module temperature would increase without limit due to its finite heat capacity, resulting in the destruction of the module.
Additionally, several other features of the C-STEPS invention described herein are innovations not identified or claimed in patent US 2010/0186794A1. These include but are not limited to:
An optically and IR transparent. GRIN or Gradium© glass aperture cover, or shaped variable thickness glass cover, used to correct for spherical aberration, in the case of a spherical primary reflector, and to advantageously redirect a large fraction of incident non-paraxial solar flux onto the secondary reflector, and thereby onto the TE module, if solar tracking is not provided;
An increasing thickness from the effective edge of the radiator/reflector toward its center to provide adequate heat transfer by conduction and thereby effect a sufficiently small radial temperature drop to ensure maximum IR radiation output; the radial distance of the effective edge of the radiator component may be may be less than the distance of the physical edge of the reflector component as determined by the optimization result of the numerical design model described herein.
Use of both sides of the reflector/radiator for maximum surface area for IR radiation to space or sky in some embodiments of the invention;
Use of an optically transparent borosilicate glass window on the hot side of the TE module to minimize IR radiation loss, beyond any such effect obtainable with the optical properties of the absorbing surface of the TE module to maximize solar absorptivity and minimize emissivity, thereby maximizing heat input to the TE module hot side to provide maximum Carnot efficiency and electricity production;
Identification of the optimum reflector system as shown in FIG. 6 for the dual reflector system, viz., two parabolic reflectors (concave primary and convex secondary) such that their foci are coincident, thereby allowing all solar flux incident onto the concave reflector and redirected to the convex reflector to be completely collimated in a symmetrical distribution onto the TE module hot side surface;
Inclusion of a variety of safeguard mechanisms in the invention design to prevent an unsafe high temperature at the TE module hot side or within the TE module proper.
Use of the invention to augment the power output of a solar panel, especially in a space application, in some embodiments of the invention.
A numerical design model to optimize the C-STEPS design geometry as a function of the constraints and input parameter requirements, and to provide the optimum optical path of incident solar radiation as a function of that design, said model examples provided in Addenda 1 and 2.